A navigation system mounted in a vehicle detects an absolute position and heading direction of a subject vehicle in a high accuracy using GPS (Global Positioning System) or the like. Based on the position of the vehicle and map data from a map database, the position and heading direction are superimposed on a map displayed on a display unit to thereby achieve a location function. Further, a recommended route to a designated destination is retrieved and indicated to a user to thereby achieve a route guide function.
In the location function, a map matching is used to superimpose the position of the vehicle on a road on the map electronically displayed. In the map matching, a vehicle's track and a road shape in the road map data are compared with each other to estimate a road where the vehicle is running. One method for the map matching can identify not only a road but also a traffic lane along which a vehicle is running (see Patent document 1). Further, a positioning accuracy (i.e., matching ratio) of a map matching is outputted to help prevent mis-recognition of a user (see Patent document 2).                Patent document 1: JP-H11-211491 A        Patent document 2: JP-H6-265364 A        
To increase a positioning accuracy in a car navigation system, map data stored in a map database is required to be improved in an accuracy as a whole and to be updated as needed. Providing a highly accurate map for an entire nation (e.g., Japan or the United States of America) involves significant maintenance costs and a possibility of exceeding data capacity for a map data storage medium (HDD, DVD).
Further, detail map data is apt to decrease reliability with time elapsing. Maintaining the reliability of map data requires frequent on-site investigations and frequent map updates. An update may be applied only to a specific segmental region without being applied to any other region. In this case, the user can use detail map data in the specific region, but not in the other region. This may make a user feel strange and make it difficult to use the map data. Thus, map data is typically stored in a map database in an accuracy maintained in an identical level in all regions within the nation.
Further, it is proposed that a vehicle position detected by a navigation system and proximity map data may be used to another driving assistance application in addition to the navigation function. For instance, the application includes a brake control to prevent a collision, a headlight control to follow a road shape such as a curve, and an air-conditioner control to meet a condition in a location. In this case, required accuracy in positioning varies depending on kinds of applications. Very high accuracy may be required to the brake control, while not so high accuracy is required to the air-conditioner control. Therefore, if the positioning accuracy in map data is maintained in an identical level in all regions, a certain driving assistance application may not stably function.